Biogeochemical approach to understanding phosphogenic events of the terminal Proterozoic to Cambrian

The Precambrian-Cambrian (PC/C) transition is one of the most remarkable in tervals in Earth history. Here, we report the S-isotopic compositions of tr ace sulfates in apatites of two phosphorite deposits spanning the PC/C boun dary on the Yangtze Platform, southern China. A comparison of these S-isoto pe data with published delta(13)C profiles from contemporaneous carbonates suggests that current views concerning phosphogenesis during this time inte rval need to be reassessed. In the case of the Late Sinian phosphogenic eve nt, which led to the formation of the Doushantuo phosphorites, an excessive development of microorganisms and the operation of the Fe-redox pump may h ave been primarily responsible for the processes of phosphorite formation. This suggests predominantly geobiodynamic controls rather than tectonic for cing. By contrast, the deposition of the Early Cambrian Zhongyicun phosphor ites was probably caused by a breakup of the pre-existing supercontinent an d a concomitant transgression, with upwelling nutrient-rich waters triggeri ng a rapid diversification of small shelly invertebrates on the newly gener ated continental shelves. (C) 2000 Elsevier Science B.V. All rights reserve d.